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Sunday, 7 February 2016

Bronzes

Bronzes:
          Bronze is an alloy of copper and tin although commercial bronzes may contain other elements besides tin. In fact, alloys of copper with aluminium, silicon and beryllium, which may contain no tin are also known as bronzes. The simplest bronze contains 88% Cu with 12% Tin.
          Tin bronzes are of a beautiful golden color. As in brasses, both tensile strength and ductility of bronzes increase gradually with increases in the amount of tin content. However, more that 20% tin is not used in bronze as it results in the formation of a brittle inter metallic compound, Cu3Sn, and also its tensile strength very rapidly decreases. Addition of tin to copper up to 10% increase the strength, hardness and durability to a much greater extent than the addition of zinc to copper.
          As cast the alloy containing 9% of the tin has a tensile strength of about 215 MN/m2.
       By alloying 2 parts copper with 1 of tin, a very hard, brittle while alloy is produced called 'Spectrum metal'. When highly polished it serves excellently for mirror and reflectors.

The following varieties of tin bronzes are commonly used:

(i) Phosphor-Bronze:
          Addition of 0.5% phosphorous to tin bronze results in production of phosphorous bronze. It is certain, however, that whatever good qualities the bronze might have, they are greatly increased by the addition of phosphorous. Phosphorous increases fluidity of moltan metal and fine castings and rolling can be made. It seems to resist corrosion better than ordinary bronze, especially by sea water, so that it is much used for propeller blades. On account of its toughness, elasticity and strength it may serve as a substitute for steel as in the manufacture of corrosion-resistant mine cables, ship sheathing, valve parts, springs etc.,
(ii) Leaded-Bronze:
          Addition of lead to tin bronze, results in production of leaded bronze. Lead is actually a source of weakness, but adds to machinability and has self lubricating properties. Usually, lead percentage does not exceed 2%.
(iii) Gun-metal:
         It contains 2% zinc, 10% tin and 88% copper. It is a very famous composition. This bronze is used for bearing bushes, glands, pumps, valves etc.,
(iv) Bell-metal:
          It is a tin bronze but having a very high percentage of tin (20–25%). It gives a good tinkling sound on being struck with a hammer. Such alloys are very hard and brittle, but are sonorous and are employed, therefore, in making bells.

Bronzes having no tin:
The following bronzes contain no tin and are commercially well known:
(i) Aluminium-bronze:
          Composition: 14% Aluminium, 3% of iron, o.5% tin(Sn), rest copper. Color: Golden yellow. It possesses good strength and good corrosion resistance. It is somewhat difficult to cast because of oxidation. It's melting point is about 1040. The commercial aluminium bronzes are practically non-magnetic, but certain complex bronzes containing appreciable proportions of iron are magnetic. Often used for costume Jewellery. It is also used in massive work, bearing metal in locomotives.
          A disadvantage of aluminium bronze is that it usually develops surface films of high electrical resistance when used for current carrying purposes; these films tend to give rise to high electrical resistance and render jointing a matter of difficulty.
(ii) Silicon-bronze:
          Composition: 1–4% Silicon, and up to 1% of manganese rest mainly copper with little amounts of tin, zinc and iron. Possesses extremely good corrosion resistance. Can be cold worked and strain-hardened. Used for boiler fitting and marine fitting as screws, belts, tubing's, pumps etc.,
(iii) Manganese- bronze:
          Composition: 40% zinc and 55–60% copper with 3–5% manganese. It is essentially a brass to which manganese has been added. It is used for ship's propellers.
(iv) Beryllium-bronze:
          It contains about 2% Be. Beryllium is very costly. It has very good mechanical properties (high yeild point and high fatigue limit) and can be cold worked and age-hardened. It has excellent cold and hot resistance. It is mainly used for bellows, bourdon gauge tubes, heavy duty electrical switches and cams etc.,
          Being sparking, it is utilized for making chisels and hammers under conditions where spark might cause an explosion.

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